% Function:
%   plot vertical line for specified x on subplot graph(2*1)
% Argument:
%   x: specified x value
%   s: line specific
% Return:
%   ln: Line handle
function ln = vlin2(x, s)
    % check arguments
    if nargin == 1
        s = 'k--';
    end

    % get axes handle
    subplot(2, 1, 1);
    l1 = vline(x, s);

    subplot(2, 1, 2);
    l2 = vline(x, s);

    ln = [l1; l2];
end
